Air/water and air/air heat exchangers of various types are, of course, known. These generally comprise plates or vanes or fins and/or pipes through which the two flows of air and/or water can be separately directed and which form respective passages or ducts for these media so that one of the media can transfer its heat to the other medium by indirect heat exchanger through the plates or fins of the heat exchanger. A high degree of heat exchanger efficiency is obtained when the two plates are passed generally in counterflow, i.e. one of the media passes from one side to the opposite side of the heat exchanger while the other medium flows from that other side toward the first-mentioned side in the respective flow passages.
All of these heat exchangers are characterized by the fact that a high degree of heat exchange can only be obtained when the heat exchanger is comparatively large. When heat exchangers are made smaller to minimize energy losses, the heat efficiency generally falls and as a result it has not been possible heretofore to satisfactorily provide a heat exchanger whose dimensions and weight are practical for all heat exchanger requirements.
Furthermore, large heat exchanger units are comparatively difficult and expensive to clean and it is commonly necessary to replace an entire heat exchanger upon the development of a defect in only a part thereof.